Friction welding is a solid-state (no melting of materials) welding process that produces a fine grain forged weld by using the heat generated between a rotating weld item under an axial load and a fixed work-piece. Heat for welding is generated by direct conversion of mechanical energy to thermal energy at the interface of the work pieces without the need for the application of electrical energy or heat from other sources.
Therefore, friction welding produces high quality welds and can be performed at temperatures below the melting point of the metals that are being bonded together. The temperature is not high enough for a liquid weld pool to form from the two component materials which also facilitates welding of dissimilar metals which may be difficult to weld by higher temperature processes such as electric arc welding.
A weld item may typically be a metal stud or plug, which may be substantially cylindrical or tapered in shape and may have an external thread or part thereof. The weld item is rotated at high speed which depends on the diameter of the weld item but is typically around 1000-10,000 rpm and forced onto a static metallic component or work-piece. Friction from the contact between the rotating metal weld item and the fixed metal component generates heat. As the temperature between the interface of the weld item and the static component increases, the strength of the metals at the interface is reduced and the materials of the weld item and the static component will flow plastically under pressure from the applied force as the temperature reaches around 700-800 degrees Celsius.
Weld items are not limited to solid cylindrical forms and may for example be hollow or tubular such as a weld nipple, and may have a polygonal external profile such as a hexagon for example. The term weld item or weld stud used throughout this description is intended to cover all such forms.
In order to carry out a friction welding operation subsea, underwater welding tools have been developed and the weld items are typically fitted with a shroud formed of an anti-quenching material, which may be a material such as a foam, which fits over the weld item and seals between the weld item and work-piece preventing the weld from being cooled rapidly by the surrounding water.
A weld item and an anti-quenching foam shroud is typically manually inserted in conjunction with the weld item, into the chuck of the welding tool at the surface or topside prior to the welding tool being carried by an ROV to the weld site. As noted above, the foam shroud acts as a barrier between the weld and the surrounding seawater, reducing the quenching of the weld from the cold water. Manually installing the weld stud and foam shroud at the surface is a difficult and time consuming operation. The weld stud which is typically threaded, is wound into a threaded chuck on the welding apparatus.
During transit of the ROV to the weld site there is a danger that the foam shroud may be dislodged or damaged such as when transiting through the splashzone from the surface to the weld site. The foam type shrouds can limit the friction welding depth due to issues with compressibility at depth which can compromise the anti-quench functionality of the shroud and could lead to an incomplete weld or failure of the weld operation using the loaded weld item. In the event that the ROV reaches the weld site and the foam shroud is compromised, it is necessary for the ROV to return to the surface and for the damaged or dislodged foam shroud to be removed and a replacement weld item, with a fresh anti-quench foam shroud, to be loaded onto the chuck of the welding tool and the ROV to transit back to the weld site, which adds time and costs to the welding operation and further risks of displacement or damage to the weld items during the repeated transits.
A control system is typically deployed to the weld site separately from the welding head which adds to the complexity of the operation and provides additional risks to the success of the operation should the control system fail or be damaged during transit.
Operating depths may be limited due to subsea currents effecting the position of the control system (deployed by wire) relative to the ROV position.
Once at the required subsea location, the ROV locates the weld head with the weld item and surrounding foam shroud into a weld clamp which is typically attached to the work-piece temporarily via mechanical clamping, suction cups or magnets. The subsea friction weld is then carried out under command instructions from the surface for example under control signals supplied by a control wire between the surface and the welding apparatus.
Completed friction welds can then be checked by carrying out a pull test where a tensile force is applied to the weld item from the weld head.
After the weld item is successfully friction welded to the work-piece, the weld head typically winds off the chuck from the threaded stud by reversing the direction of rotation of the weld head which allows the threaded portion of the weld item and the threaded chuck of the weld head to disengage. This winding off operation can damage the threads of the stud.
The weld head is then disengaged from the weld clamp and the ROV is recovered to the surface where the next weld stud and protective foam shroud is loaded before the ROV returns to the weld site for the next stud to be welded.
The present invention aims to provide a welding apparatus and method that enables the welding of consecutive weld items at the required subsea location with an increased operating depth without having to recover the welding apparatus to the surface between welds.